The invention relates to a transistor amplifier having a first transistor pair, which comprises a first and a second transistor, and having a second transistor pair, which comprises a third and a fourth transistor, in which the transistors of each pair have their emitter electrodes coupled to each other and to an associated constant current source, the first transistor has its base electrode and its collector electrode coupled to the corresponding electrodes of the third transistor, and the second transistor has its base electrode and its collector electrode coupled to the corresponding electrodes of the fourth transistor, the base electrodes of the first and the third transistor, on the one hand, and the base electrodes of the second and the fourth transistor, on the other hand, being arranged to receive an input signal and an output signal being derivable from the collector electrodes of the transistors, and having a first, a second, a third and a fourth negative-feedback circuit each included in the coupling to the associated constant current source.
Differential amplifiers using bipolar circuit technology are generally known. As a rule, a differential amplifier of this kind is a voltage-controlled current source formed by means of two transistors whose emitter electrodes are connected together to a constant-current source. The base electrodes of the transistors then form a differential voltage input and the collector electrodes form a differential current output. The transfer function between the output current and the input voltage is a hyperbolic tangent function. The derivative of this function with respect to the input voltage exhibits a maximum when the input voltage becomes zero. The deviation of this derivative from the maximum value is a measure of the degree of linearity of the differential amplifier. For the described simple differential amplifier the transfer function exhibits a linearity error of 7% when said amplifier is driven with an input voltage of 15 mV (positive or negative). Such a degree of linearity is inadequate to achieve small-error signal processing in the case of comparatively large inputs. However, adding further elements in order to improve the linearity of the described differential amplifier leads to more component parts and to more noise.
DE-OS 30 27 071 discloses a transistor amplifier having a first transistor pair comprising a first and a second transistor as well as a transistor pair comprising a third and a fourth transistor. The transistors of each pair have their emitter electrodes interconnected. Moreover, the first transistor has its base electrode and its collector electrode to the corresponding electrodes of the third transistor and the second transistor has its base electrode and its collector electrode coupled to the corresponding electrode of the fourth transistor. The base electrode of the first and the third transistor, on the one hand, and the base electrode of the second and the fourth transistor, on the other hand are each coupled to a terminal of a low-impedance signal source. The output signal of the transistor amplifier can be derived from the collector currents of the transistors. The common emitter lines of the two transistor pairs include corresponding constant current sources. The transistors are implemented and/or operated in such a manner that the collector direct currents through the first and the fourth transistor are equal to one another and are a factor k of between 2 and 30 times as large or as small as the mutually equal collector direct currents through the second and the third transistor. This is achieved specifically in that the second and the third transistor are identical to one another and each have an emitter area which is a factor k as large as the emitter area of the first and the fourth transistor, which are also identical to one another. Enlarging the emitter areas by a factor k causes the emitter direct current or the collector direct current through the respective transistor to be increased by the factor k. When the value chosen for the factor k is 4, the input voltage for a given third-order harmonic distortion can be around five times as high as the input voltage of a single transistor pair having a constant current source in the common emitter line when the direct currents through the two transistors, i.e. the ratio of their areas, are equal.
Moreover, it is known from DE-OS 30 27 071 to arrange split negative-feedback resistors between the emitter electrodes of the transistor pairs and to connect the constant current sources to the resistor taps. Such a negative-feedback circuit produces larger noise signals and has a lower gain but can also provide a linear amplification for large signals.
From EP 0 352 790 B1 an integrator circuit is known which includes emittercoupled transistor pairs. In these transistor pairs the emitter electrodes of the transistors are each connected to one another and to constant current sources via a diode poled in the forward direction. By means of the diodes the swing of the input signal is substantially doubled in comparison with an integrator circuit without said diodes while the linearity of the integrator circuit is maintained.
From U.S. Pat. No. 3,855,541 it is known that a series arrangement of a given number of diode-coupled transistors which each have an effective emitter area of a size m can be replaced with a transistor circuit including a series arrangement of the same number of diode-coupled transistors but whose effective emitter areas are reduced by the factor m. Moreover, a further transistor has its collector-emitter path arranged in parallel with the series arrangement of transistors in the last-mentioned circuit and has an effective emitter area which is (m-1) times as large as the effective emitter area of each of the transistors of the series arrangement with which said further transistor is arranged in parallel. Furthermore, said further transistor has a base electrode coupled to the base electrode of that transistor of the series arrangement which has its emitter electrode connected to the emitter electrode of the transistor arranged in parallel with the series arrangement.